Coral Reefs https://doi.org/10.1007/s00338-021-02137-3

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Linear extension, skeletal density, and calcification rates of the Heliopora coerulea

1 2 2 Travis A. Courtney • James R. Guest • Alasdair J. Edwards • Romeo M. Dizon3

Received: 25 January 2021 / Accepted: 10 June 2021 Ó The Author(s) 2021

Abstract The brooding reef-building octocoral Heliopora Keywords Extension Á Skeletal density Á Calcification is widespread on Indo-West Pacific reefs and appears to be rate Á Blue coral Á Octocoral Á Ocean warming Á Ocean relatively resistant to thermal stress, which may enable it to acidification Á Coral reef restoration persist locally while scleractinians diminish under Anthropocene conditions. However, basic physiological measurements of ‘‘blue corals’’ are lacking and prevent Introduction their inclusion in trait-based studies. We address this by quantifying rates (mean ± SE) of linear extension Calcium carbonate production represents one of the core (0.86 ± 0.05 cm yr-1) and skeletal density functions of coral reef ecosystems (Brandl et al. 2019) and (2.01 ± 0.06 g cm-3) to estimate calcification rates can be approximated from estimates of benthic cover and (0.87 ± 0.08 g cm-2 yr-1) for the small branch- taxa-specific calcification rates as part of census-based ing/columnar morphology of Heliopora coerulea.We coral reef carbonate production budgets (Perry et al. 2012). postulate that H. coerulea may become an increasingly While notable efforts have advanced estimates of carbonate important reef-builder under ocean warming due to its production states through the aggregation of publicly relative resistance to thermal stress and high skeletal den- accessible coral linear extension, skeletal density, and sity that make colonies less vulnerable to storm damage calcification rate data (e.g., Perry et al. 2012; Madin et al. under ocean acidification. Moreover, Heliopora corals are 2016), basic physiology measurements of the reef-building likely dispersal limited suggesting they may be an under- octocoral Heliopora are largely absent from these appreciated genus for restoration of stress-tolerant reef- databases. To the best of our knowledge, these are limited building capacity on degraded reefs. to rates of lateral overgrowth of 2.1 ± 0.9 cm yr-1 for H. coerulea over Porites spp. corals reported by Guzman et al. (2019) and a solitary H. coerulea calcification rate estimate of 0.48 g cm-2 yr-1 (by Planck et al. 1988 in Ryan et al. 2019) in the scientific literature. Widely distributed across the Indo-Pacific, Heliopora & Travis A. Courtney corals are distinct in that they represent the only known [email protected] genus of hermatypic octocorals (Zann and Bolton 1985). & James R. Guest Often termed ‘‘blue corals’’ owing to the incorporation of [email protected] iron salts into their skeletal aragonite, Heliopora are gen- 1 Scripps Institution of Oceanography, University of erally typified by their iconic blue skeletons (Hill 1960; California, La Jolla, San Diego, CA, USA Richards et al. 2018). Heliopora coerulea was considered 2 School of Natural and Environmental Sciences, Newcastle the only extant species, but recent genetic evidence has University, Newcastle upon Tyne, UK suggested cryptic speciation within H. coerulea between 3 Department of Biology, University of the Philippines Baguio, small and flat branching morphologies (Yasuda et al. 2014) Baguio City, Philippines that have asynchronous reproductive timing (Villanueva 123 Coral Reefs

2016). Heliopora hiberniana was also recently described in fragments represented the small branching/columnar mor- north Western Australia (Richards et al. 2018). Interest- phology sensu Yasuda et al. (2014) and Villanueva (2016) ingly, Heliopora appears to have higher optimal tempera- (Fig. 1a). Fragments were sourced from 25 independent ture ranges compared to many scleractinian corals as colonies (four fragments per colony) and epoxied (Pioneer evidenced by apparent range retractions during periods of Epoxyclay AquaTM) onto replicate calcium carbonate cooling (Zann and Bolton 1985), high degree of bleaching substrates (locally aquacultured Tridacna gigas shells) resistance and resilience during recent thermal stress events mounted on plastic mesh frames between mid-December (Kayanne et al. 2002; Donner et al. 2010; Phongsuwan and 2005 and mid-January 2006 in five replicate plots at Changsang 2012; Harii et al. 2014; Richards et al. 2018; 2.9–3.4 m depth within the lagoon north of Silaqui Island, Guzman et al. 2019; Ryan et al. 2019), apparent increasing Philippines (16°26022.000N 119°56037.800E). All linear competitive advantage in warmer waters (Atrigenio et al. extension data presented here were collected as part of a 2020), and growth rates that increase with seawater tem- larger experiment to examine coral growth and survival peratures up to at least 31 °C (Guzman et al. 2019). H. rates (e.g., see Guest et al. (2011) for further details on coerulea also contributed to the maintenance of positive study design and photographs of outplanted corals). Linear coral reef carbonate production states following a coral extension was determined from repeated caliper (preci- bleaching event (Ryan et al. 2019), providing further evi- sion = 0.1 mm) measurements of fragment height on six dence that H. coerulea may become increasingly important occasions in February, May, and October 2006, April and reef-builders under ongoing ocean warming. September 2007, and finally in May 2008. Over the * Owing to the absence of basic physiology data for He- 2.25 years of the study, none of the 100 fragments suf- liopora corals, it remains unclear how the increasing rel- fered mortality, but two were either broken or became ative abundance of Heliopora corals may impact coral reef unattached, so only prior growth was recorded. Linear geo-ecological functions under ongoing ocean warming extension (cm yr-1) was estimated from the least squares and acidification. In this study, we quantify linear exten- regression line slope of height (cm) vs. time (yr) from each sion and skeletal density to estimate morphology-specific survey for each fragment to make use of the six repeated calcification rates to fill these gaps in the trait-based coral height measurements for each fragment and minimize literature and speculate as to the role of Heliopora coerulea effects of individual measurement error. as an emerging reef-builder and potential restoration target in the Anthropocene. Skeletal density

In the absence of direct, contemporaneous skeletal density Methods measurements on the outplanted Heliopora coerulea frag- ments, we used three specimens collected from the Fed- Linear extension erated States of Micronesia in 1967 and preserved in the Scripps Institution of Oceanography Benthic Invertebrate Large and small Heliopora coerulea fragments (n = 100) Collection (i.e., SIO-BIC Co151, Co276, Co277) (Fig. 1b) of mean ( ± SE) initial planar area of 22.3 ± 1.5 cm2 to quantify H. coerulea skeletal density and estimate cal- (n = 50 large fragments) and 5.2 ± 0.2 cm2 (n = 50 small cification rates. Archimedes’ principle was used to deter- fragments) were used to estimate linear extension rates mine bulk skeletal densities (g cm-3) from the mass of from repeated measures of height through time. All skeletal material (g) divided by the volume of water dis- placed (cm3) (Jokiel et al. 1978; Morgan and Kench 2012). The mass of each skeletal fragment was determined via direct measurement of the dry skeleton. The volume of displaced water for each respective skeletal fragment (cm3) was determined via the seawater equation of state (i.e., R package seacarb; Gattuso et al. 2020) using the mass (g), temperature (°C), and salinity (ppt) of the displaced water upon submergence of the skeleton in deionized water. We assumed that H. coerulea skeletal voids were small and Fig. 1 a Heliopora coerulea from Bolinao, Philippines showing the poorly connected with minimal influence on bulk skeletal typical light brown coloration of living colonies. Photograph by JR density measurements and therefore did not dip the speci- Guest. b Heliopora coerulea skeletal specimen from the Scripps Institution of Oceanography Benthic Invertebrate Collections (SIO- mens in paraffin wax prior to displacement in water BIC Co276) showing the iconic blue skeletal coloration. Photograph (Bucher et al. 1998). Mass was determined via an Ohaus by TA Courtney Scout SPX422 balance (precision = ± 0.01 g) that was 123 Coral Reefs verified for accuracy with a calibration weight prior to Skeletal density measurements. A YSI Pro 2030 was used to measure temperature (precision = ± 0.3 °C) and salinity (preci- Mean ( ± SE) skeletal densities for Heliopora coerulea of sion = ± 0.1 ppt). Skeletal densities were compared to 2.01 ± 0.06 g cm-3 in this study were higher than any of recent bulk skeletal densities of Indian Ocean scleractini- the scleractinian skeletal densities observed by Morgan and ans determined using analogous methods (Morgan and Kench (2012) (Fig. 2). Ocean acidification (OA) threatens Kench 2012). to reduce scleractinian coral skeletal density, which could lead to increased breakage of coral skeletal materials and Calcification rates decreased reef-building capacity (Hoegh-Guldberg et al. 2007). Whether distinct differences in the skeletal mor- The scaling of coral calcification rates represents a con- phology of H. coerulea may confer some resilience to OA siderable ongoing challenge owing to the three-dimen- by these reef-building octocorals remains to be tested sional structural complexity of coral skeletons, so we used (Atrigenio et al. 2020). Regardless, the comparatively methods analogous to Morgan and Kench (2012) to directly higher skeletal density of H. coerulea relative to its scler- compare our estimated calcification rates to those of that actinian morphological counterparts suggests that H. study. The small branching/columnar H. coerulea speci- coerulea skeletons could be more resistant to such physical mens in this study most closely approximated the Pocil- breakage under OA. However, we cannot rule out the lopora meandrina submassive morphology from Morgan possibilities that slight differences in methodologies (i.e., and Kench (2012), so we used the adjustment coefficient of omission of paraffin wax dip in this study) or that Helio- 0.5 for that species to account for the open space in our pora skeletal densities were indeed higher in 1967 and estimates of H. coerulea. We therefore estimated mean have since decreased under ongoing environmental change, ( ± SE) H. coerulea calcification rates from the product of so further research remains necessary to test any hypoth- linear extension, skeletal density, and the 0.5 adjustment esized resiliency of Heliopora corals and their skeletal coefficient following Morgan and Kench (2012). components to environmental change.

Calcification rates Results and discussion Mean ( ± SE) estimated calcification rates for Heliopora Linear extension coerulea were 0.87 ± 0.08 g cm-2 yr-1 for the small branching/columnar morphology investigated in this study Mean ( ± SE) linear extension did not differ significantly (Fig. 2). This rate is greater than the solitary calcification between the large (0.88 ± 0.08 cm yr-1) and small rate estimate of 0.48 g cm-2 yr-1 for H. coerulea by (0.83 ± 0.06 cm yr-1) fragments (two-sample t-test, Planck et al. (1988) in Ryan et al. (2019), which may in p = 0.531), so we calculated the mean ( ± SE) linear part be due to differences in methodologies and/or mor- extension rate for all H. coerulea fragments (n = 100). phologies between the previous measurement and this This rate (0.86 ± 0.05 cm yr-1) was lower than the study. Notably, H. coerulea calcification rates are less than extension rates of 2.1 ± 0.9 cm yr-1 reported for H. those for branching, corymbose, digitate, and massive coerulea laterally overgrowing massive Porites spp. by scleractinians, in general agreement with rates for sub- Guzman et al. (2019). It is unlikely that environmental and/ massive morphologies, and are greater than mushroom and or genotypic controls caused these differences in growth encrusting scleractinian calcification rates by Morgan and rates (Pratchett et al. 2015) as the data were collected from Kench (2012) (Fig. 2). While variability in calcification approximately the same study locations (Bolinao, Philip- rates through space and time (Pratchett et al. 2015) con- pines). We posit that these differences were most likely due found a more direct comparison of calcification rates to measurements of linear extension of skeletal branches between taxa, these estimates nonetheless suggest that (this study) versus lateral extension of a thin veneer of offsetting slower linear extension and higher density encrusted skeletal material over massive Porites spp. skeletal material of H. coerulea generate carbonate pro- (Guzman et al. 2019). Our mean ( ± SE) extension rate of duction rates similar to other reef-building scleractinian 0.86 ± 0.05 cm yr-1 for H. coerulea is less than the taxa. branching, corymbose, digitate, and massive scleractinians, similar to submassive scleractinians, and greater than mushroom and encrusting scleractinians observed by Morgan and Kench (2012) (Fig. 2).

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Fig. 2 Mean ( ± standard error) linear extension, skeletal density, and calcification rates for Heliopora coerulea from this study are plotted relative to Indian Ocean scleractinian corals from Morgan and Kench (2012)

Implications for reef building and restoration inhibit growth and/or recruitment by scleractinian corals in the Anthropocene (Atrigenio et al. 2017; Guzman et al. 2019; Atrigenio et al. 2020) and should therefore critically evaluate any potential Owing to their higher degree of resilience to thermal stress resulting consequences as part of a broader decision-mak- anomalies and apparent increasing ability to outcompete ing framework (National Academies of Sciences 2019). scleractinians under ocean warming, Heliopora corals may We conclude that Heliopora corals warrant further research become increasingly abundant reef-builders (Kayanne et al. as potentially emerging reef-builders and target taxa for 2002; Donner et al. 2010; Phongsuwan and Changsang restoration and adaptation efforts in the Anthropocene. 2012; Harii et al. 2014; Richards et al. 2018; Guzman et al. 2019; Ryan et al. 2019; Atrigenio et al., 2020). Interest- Supplementary InformationThe online version contains supplementary material available at https://doi.org/10.1007/s00338- ingly, Heliopora tends to form dense aggregations and may 021-02137-3. be dispersal limited owing to benthic planulae that settle near the parent colony (Zann and Bolton 1985; Harii et al. Acknowledgements We thank Dr. Charlotte Seid for loaning He- 2002). We posit that outplanting of stress-tolerant reef- liopora coerulea specimens from the Benthic Invertebrate Collection of Scripps Institution of Oceanography and three anonymous building Heliopora corals to degraded reef systems within reviewers for improving this manuscript with their constructive its current geographic range may enable restoration of comments. TAC was supported by funding from the National structural complexity and calcium carbonate production to Oceanographic and Atmospheric Administration Ocean Acidification select reefs. However, such restoration strategies could Program. All linear extension and skeletal density data are publicly accessible via the electronic supplementary materials. On behalf of all

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